Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
Although photopolymer printing elements are typically used in “flat” sheet form, there are particular applications and advantages to using the printing element in a continuous cylindrical form, as a continuous in-the-round (CITR) photopolymer sleeve. CITR photopolymer sleeves add the benefits of digital imaging, accurate registration, fast mounting, and no plate lift to the flexographic printing process. CITR sleeves have applications in the flexographic printing of continuous designs such as in wallpaper, decoration and gift-wrapping paper, and other continuous designs such as tablecloths, etc. CITR sleeves enable flexographic printing to be more competitive with gravure and offset on print quality.
A typical flexographic printing plate as delivered by its manufacturer, is a multilayered article made of, in order, a backing or support layer, one or more unexposed photocurable layers, a protective layer or slip film, and a cover sheet. A typical CITR photopolymer sleeve generally comprises a sleeve carrier (support layer) and at least one unexposed photocurable layer on top of the support layer.
It is highly desirable in the flexographic prepress printing industry to eliminate the need for chemical processing of printing elements in developing relief images, in order to go from plate to press more quickly. Processes have been developed whereby photopolymer printing plates are prepared using heat and the differential melting temperature between cured and uncured photopolymer is used to develop the latent image. The basic parameters of this process are known, as described in U.S. Pat. Nos. 5,279,697, 5,175,072 and 3,264,103, in published U.S. patent publication Nos. U.S. 2003/0180655, and U.S. 2003/0211423, and in WO 01/88615, WO 01/18604, and EP 1239329, the teachings of each of which are incorporated herein by reference in their entirety. These processes allow for the elimination of development solvents and the lengthy plate drying times needed to remove the solvent. The speed and efficiency of the process allow for use of the process in the manufacture of flexographic plates for printing newspapers and other publications where quick turnaround times and high productivity are important.
The photopolymer layer allows for the creation of the desired image and provides a printing surface. The photopolymers used generally contain binders, monomers, photoinitiators, and other performance additives. Photopolymer compositions useful in the practice of this invention include those described in U.S. patent application Ser. No. 10/353,446 filed Jan. 29, 2003, the teachings of which are incorporated herein by reference in their entirety. Various photopolymers such as those based on polystyrene-isoprene-styrene, polystyrene-butadiene-styrene, polyurethanes and/or thiolenes as binders are useful. Preferable binders are polystyrene-isoprene-styrene, and polystyrene-butadiene-styrene, especially block co-polymers of the foregoing.
The composition of the photopolymer should be such that there exists a substantial difference in the melt temperature between the cured and uncured polymer. It is precisely this difference that allows the creation of an image in the photopolymer when heated. The uncured photopolymer (i.e., the portions of the photopolymer not contacted with actinic radiation) will melt or substantially soften while the cured photopolymer will remain solid and intact at the temperature chosen. Thus the difference in melt temperature allows the uncured photopolymer to be selectively removed thereby creating an image.
The printing element is selectively exposed to actinic radiation. Selective exposure is generally accomplished in one of three related ways. In the first alternative, a photographic negative with transparent areas and substantially opaque areas is used to selectively block the transmission of actinic radiation to the printing plate element. In the second alternative, the photopolymer layer is coated with an actinic radiation (substantially) opaque layer, which is also sensitive to laser ablation. A laser is then used to ablate selected areas of the actinic radiation opaque layer creating an in situ negative. The printing element is then flood exposed through the in situ negative. In the third alternative, a focused beam of actinic radiation is used to selectively expose the photopolymer. Any of these alternative methods is acceptable, with the criteria being the ability to selectively expose the photopolymer to actinic radiation thereby selectively curing portions of the photopolymer.
Once the photopolymer layer of the printing element has been selectively exposed to actinic radiation, it is then ready to develop using heat. As such, the printing element is generally heated to at least about 70° C. The exact temperature will depend upon the properties of the particular photopolymer being used. However, two primary factors should be considered in determining the development temperature:                1. The development temperature is preferably set between the melt temperature of the uncured photopolymer on the low end and the melt temperature of the cured photopolymer on the upper end. This will allow selective removal of the photopolymer thereby creating the image.        2. The higher the development temperature, the quicker the process time will be. However, the development temperature should not be so high as to exceed the melt temperature of the cured photopolymer or so high that it will degrade the cured photopolymer. The temperature should be sufficient to melt or substantially soften the uncured photopolymer thereby allowing it to be removed.        
Once the heated printing element has been developed, uncured photopolymer can be melted or removed. In most instances, the heated printing element is contacted with a material that will absorb or otherwise remove the softened or melted uncured photopolymer. This removal process is generally referred to as “blotting”. Blotting is typically accomplished using a screen mesh or an absorbent fabric. Either woven or non-woven fabric is used and the fabric can be polymer based or paper, so long as the fabric can withstand the operating temperatures involved. In most instances, blotting is accomplished using rollers to bring the material and the heated printing plate element into contact.
U.S. Pat. No. 5,175,072 to Martens, the subject matter of which is herein incorporated by reference in its entirety, describes the removal of uncured portions of the photopolymer by using an absorbent sheet material. The uncured photopolymer layer is heated by conduction, convection, or other heating method to a temperature sufficient to effect melting. By maintaining more or less intimate contact of the absorbent sheet material with the photocurable layer, a transfer of the uncured photopolymer from the photopolymer layer to the absorbent sheet material takes place. While still in the heated condition, the absorbent sheet material is separated from the cured photopolymer layer in contact with the support layer to reveal the relief structure. After cooling, the resulting flexographic printing plate can be mounted on a printing plate cylinder.
Upon completion of the blotting process, the printing plate element is preferably post-exposed to further actinic radiation, cooled and then ready for use.
Depending upon the particular application, the printing element may also comprise other optional components. For instance, it is frequently preferable to use a removable coversheet over the photopolymer layer to protect the layer during handling. If used, the coversheet is removed either just before or just after the selective exposure to actinic radiation. Other layers such as slip layer or masking layers as described in U.S. Pat. No. 5,925,500 to Yang et al., the teachings of which are incorporated herein by reference in their entirety, can also be used.
The drawback to many of the current methods of thermal development is that these methods can be used only on flat plates, which must then be mounted after development. This operation requires an additional machine and more time and can also result in a loss in accuracy when registration between multiple plates and colors is required.
Current thermal development apparatuses using heated rolls for blotting away the uncured photopolymer typically use only one heated roll that is of approximately the same width as the plate. This increases the difficulty in making printing elements of different sizes. In addition, additional problems may arise when attempting to make the blotting machine larger.
Another problem with the current blotting methods is that a tremendous amount of force (approximately 100 pounds/linear inch) must be applied by the heated roll to force the blotting material into the image on the printing element. This large force can cause the heated roll to bend, resulting in an uneven floor. Furthermore, the heating and blotting process must often be repeated several times in order to obtain effective removal of the uncured photopolymer.
While various thermal development apparatuses have been developed for use in the production of relief image printing plates, there remains a need in the art for a thermal developing device that can be used with cylindrical printing elements, either as continuous printing sleeves or as plates that are then formed into printing sleeves. Furthermore, there remains a need in the art for a thermal developing device that can easily be used to produce different sized printing elements.